In the field of electromagnetic and optical radiation, retroreflectors are well known devices which are often used as transponders. Retroreflectors receive electromagnetic energy and reflect such energy back in the direction from whence it came. A passive retroreflector returns a signal with the same general characteristics of the incident signal, preferably with relatively high directional gain and relatively little spreading of the incident beam. In contrast, an active retroreflector may include an emitting device that can generate a user definable signal in response to the incident radiation beam.
Retroreflectors have found a wide variety of applications in numerous fields including communications systems, monitoring systems, and in-flight refueling systems. Examples of different types of retroreflector structures include a corner-cube reflector, a horn reflector, a parabolic dish reflector, and a parabolic cylinder reflector. Illustrations of these and other retroreflector configurations can be found in numerous publications including, for example, U.S. Pat. No. 4,517,569 to Gerharz. Another type of well known retroreflector is a cassegrain reflector, as disclosed by Gordan E. Peckham and Robert A. Suttie in "Microwave Reflection Properties of a Rotating Corrugated Metallic Plate Used as a Reflection Modulator, " IEEE Transactions on Antennas and Propagation, Vol. 36, No. 7, pp. 1000-1006 (July 1988).
Yet another application of retroreflectors is in the task of identifying friend-or-foe (IFF) in a battleground setting. Since the evolution of weaponry which allowed opposing forces to fight through the exchange of the instrumentalities of war at a distance, fratricide killing has been a problem. IFF tasks are a delicate comprise between secure, ambiguous identification and the maintenance of stealth positions. In typical IFF systems, a radio or microwave frequency request is made by an interrogation unit such as a plane or tank and a corresponding signal is returned by the targeted unit. This is normally achieved by a transponder on the targeted unit that emits a coded return signal when the interrogation request is received. Other systems merely re-radiate or reflect the incident interrogation request, while some systems modulate the re-radiated or reflected signal in an distinctive manner. The interrogation unit then deciphers the received signal to determine if the targeted unit is a friend or foe. However, by emitting (i.e., reflecting) a broadly directed response that is designed to have a sufficient strength to reach the interrogation unit, some of the radiation may be detected by other units of the opposing force which may reveal the position of the targeted unit.
Examples of re-reflector systems utilized in IFF tasks are discussed in U.S. Pat. No. 4,361,911 to Buser et al., and U.S. Pat. No. 5,274,379 to Carbonneau et al. The patent to Buser et al is directed to a laser retroreflection system with a high power laser interrogator and a dedicated receiver that sends a laser pulse with a cryptic interrogation message that can be decoded by a friendly target. The friendly target has a retroreflector that reflects the laser light only when the correct preselected cryptic interrogation message is detected. An acousto-optic modulator modulates the reflected signal with a preassigned intensity modulation. If the modulation code dedicated by the receiver of the laser interrogator is correct, the target is deemed friendly. However, the system of Buser et al. requires over 100 watts to operate, which in even short burst modes may be too high for many light weight applications. In addition, residual radio frequency (RF) leakage may make an acousto-optic device such as the one disclosed in Buser et al. easily detectable by hostile forces. Moreover, it appears that the system of Buser et al. may require precise spatial registration of the source lens and the receiver lens to ensure retroreflection of the incident laser pulse.
The patent to Carbonneau et al. is directed to an optical system wherein all vehicles are provided with a radiation transmitter and a receiver. The receiver includes a detector for detecting radiation transmitted by other vehicles. When a vehicle receives and correctly identifies a coded signal from another friendly vehicle, an unblocking signal is produced to clear the radiation transmission path, thereby allowing a reflector to reflect the received signal back to the source of the transmission. The reflector further adds a predetermined code to the reflected signal so that the vehicle receiving the reflected signal can identify the further predetermined code. The reflector in Carbonneau et al. is a retroreflector that utilizes a rotating disk to add the predetermined code to the reflected signal. However, because the information encoded on the reflected signal depends upon the disk, the total amount of information which can be transmitted is limited by the particular disk utilized, and how fast the disk can be made to rotate on the spindle.
Therefore, an unsatisfied need exists in the industry for a low power reflector system capable of providing stealth communications.